It is common in the field of agriculture to make use of automation systems to help steer tractors and other agricultural land-working vehicles such as harvesters, sprayers and their attached implements. All of these types of farm equipment are referred to hereinafter as “mobile agricultural machines.” In the past, farm vehicles and guided implements have made use of sensors to help guide them along pre-defined or pre-existing swaths or rows. The swath width is the coverage width of the implement in use, and for crops planted in rows it is an integer multiple of the row spacing. These sensors, which will be referred to herein as path sensors, measure the offline distance of the vehicle or implement to the crop rows, ground furrows or edge of previously harvested crop swath using either contact sensors such as suspended weights or wands, or using non-contact sensors (laser, sonar, optical imaging). These locations determined by the path sensor are referred to as path reference points. Path sensors include sensors that measure distance to crop rows, either the plants themselves or ground features of the rows, such as ruts between the beds of the planted rows. In particular, swath sensors measure distance to the edge of a swath, such as the distance to the edge of the previously harvested adjacent swath. Thus, the path sensors include both row and swath sensors and typically sense physical features along a vehicle path such as furrows in the ground or crops along the sides of a swath.
One type of prior art furrow sensor is shown in FIG. 1 and comprises one or more weights 100 dragging on the bottom of the furrow under the farm vehicle or implement and connected to sensors that indicate when the weight is either on the left or right hand side of the centre of a furrow in a ploughed field. FIG. 2 shows an example of a swath sensor as known in the prior art, in this case defined by a contact sensor in the form of two wands 200 extending outwardly and arranged to brush against crops planted in proximity to the vehicle path. The magnitude of the deflection or relative deflection of adjacent sensors gives a measurement of the deviation from a reference line, e.g., as defined by a furrow, a row of crop stalks, etc. (the deviation from the reference line also being referred to herein as the offline distance of the vehicle/implement.). In the case of wand sensors, the peaks in the deflections can be used to make measurements to the crop stalks, by translating the peak measurement voltage into a peak deflection angle and thus into an offline distance. Similarly the relative deflections of weights arranged side-by-side, such as the weights 100 shown in FIG. 1 can be used to indicate the location of a vehicle or implement relative to a furrow.
The functioning of a path sensor is depicted generally by the adjustable arm sensors shown in FIGS. 3 to 5, which show a pair of pivotal arms 300 connected to sensors, and the resultant signal output in the form of right and left arm signal outputs 3010 when the vehicle is either correctly aligned relative to a desired path (FIG. 3) or deviates to the left or right (FIGS. 4 and 5, respectively).
One major problem is when there is a gap in the information from the path sensors, for instance due to a gap in the planted row of crops due to a failure of some of the seeds to germinate. In this situation the sensor is either unable to produce useful offline distance measurement for vehicle guidance or it simply produces erroneous measurements. Another problem when using the path sensors alone is that absolute position, heading and speed information is not available.
Another disadvantage of path sensors is that many of them have a limited measurement range and may be affected by the heading of the vehicle/implement relative to the path. This becomes particularly problematic when the vehicle has to steer to the next swath after a turn around from one swath to the next since the physical path references are typically interrupted at the ends of the fields.
A more recent approach to auto-control of agricultural vehicles is the use of Global Navigation Satellite Systems (GNSS) such as Global Positioning System (GPS), differential GPS (DGPS) or Real Time Kinematic (RTK) techniques, with or without inertial sensors. Differential GPS (DGPS) improves results over pure GPS by providing corrections to the GPS receiver that are used by the GPS receiver to improve the accuracy of the position fix. The corrections are generated at a reference station or a network of reference stations, and broadcast via Coast Guard transmitters or satellite transmitters as part of the Wide Area Augmentation Service of the Federal Aviation Administration. DGPS thus enables the GPS receiver to achieve a higher degree of accuracy (typically about 20 cm) than is possible with pure GPS, which is limited to an accuracy of about 4 meters. For even greater accuracy, Real Time Kinematic (RTK) techniques developed for the survey market are used. The RTK method enables the GNSS receiver to eliminate more errors that degrade the position accuracy of an uncorrected GNSS receiver system. The RTK method enables the GNSS receiver to calculate a vector distance (range and bearing) from a reference receiver, or from a chosen reference point. The latter point is referred to as a Virtual Reference Station. However, even these enhancements to GPS leave undesirably large errors when considering the size and separation of typical furrows in a prepared field.
In addition to above-mentioned errors or inaccuracies in the GNSS system, there are off-sets that are introduced due to mechanical shortcomings of the vehicles and implements, e.g., failure of a towed implement to correctly track the path of the tractor pulling it. These will be referred to as mis-registration. Thus even if all GNSS errors could be eliminated, the mis-registrations would introduce off-sets. For instance, it is common to find that fields have been prepared using manual systems, path sensor systems, or with towed implements which may drift with respect to the tractor due to field irregularities resulting in crop position shift. The result is that subsequent working of the field, such as harvesting or spraying with the use of a satellite control system such as RTK requires repeated manual interventions to take into account discrepancies in the actual path compared to the path defined by using the satellite system. In addition to mis-registrations and GNSS errors, physical characteristics of the land, e.g., obstructions such as trees or boulders, may force a vehicle path to deviate.
Four deviation conditions can be defined as a result of the above problems. These are illustrated in FIGS. 6 to 9. Typically a starting line A-B is defined, also referred to as a guidance path as shown in FIG. 6, which is then used as a reference for subsequent paths formed relative to the A-B reference as the farm vehicle moves back and forth across the field. FIG. 6 illustrates a varying width 600 between rows (which may have been caused by mis-registrations between tractor and plough at time of plowing or by errors in the GNSS measurements if a satellite system was originally used to define the rows). Therefore, subsequent working of the field using a satellite guidance system requires the driver to adjust or “nudge” the position of his vehicle at the start of each row.
FIG. 7 illustrates actual wavy row pattern e.g., due to geographic undulations, as indicated by reference numeral 700 compared to the ideal straight line 702. Instead, this may be caused where a field is actually be laid out with straight parallel paths to start with but due to GNSS errors on a satellite based system used in subsequent operations, the vehicle path defines a wavy pattern as indicated by the wavy lines 700.
Yet another deviation is indicated in FIG. 8, where the paths 800 veer outward or inward relative to each other rather than remaining parallel, thus defining a first order function.
FIG. 9 shows another condition requiring manual intervention. In this case an obstacle 900 such as a tree or boulder makes it necessary for the vehicle to divert from its preferred parallel path requiring the driver to make repeated adjustments on subsequent paths to take account of the diversion.
The present invention seeks to address the problems discussed above and thereby alleviate the impact on the driver. The present invention also seeks to reduce the cost of providing an auto steering solution.